The proposed experiments will investigate two questions: 1) What are the normal developmental changes which establish the mature form and size of axon terminal arbors in the central nervous system?, and 2) Is the normal development of individual axonal projections dependent upon normal levels of sensory stimulation, and, if it is, when and in what manner is this dependence manifested? These experiments will employ as a model the axonal arbors that connect second-order auditory neurons in the avian nucleus magnocellularis with the dendrites of third-order auditory neurons in nucleus laminaris. Methods for maintaining whole chicken brain stems in vitro, for obtaining intracellular physiological recordings from n. magnocellularis neurons, and for filling these neurons and their axonal processes in n. laminaris with horseradish peroxidase will be used. The normal development of individual axonal projections will be quantitatively described and the effects of sensory stimulation and deprivation during development on individual axonal projections will be assessed in both "competitive" and "non-competitive" situations.